Conventionally, as a low-heat-loss stabilized power supply which operates with comparatively high efficiency in a large input-output difference condition, switching regulators have been widely used which generate a desired output voltage from an input voltage by driving an energy storage device (a capacitor, an inductor, or the like) by turning on and off (controlling the duty of) an output transistor.
A conventionally common switching regulator includes an error amplifier which amplifies the voltage difference between a feedback voltage Vfb which varies depending on an output voltage Vout and a predetermined reference voltage Vref, and turns on and off an output transistor by using an output signal (an error voltage Verr) of the error amplifier. More specifically, such a switching regulator generates a PWM (pulse width modulation) signal having a duty corresponding to the result of comparison between the error voltage Verr and a predetermined slope voltage Vslp (a triangular wave or a ramp wave), and turns on and off the output transistor by using the PWM signal (see, for example, Patent Publication 1 listed below, which was filed by the applicant of the present application).
The conventional switching regulator described above also includes a soft-start circuit as means for preventing an overcurrent from flowing into the load at the start-up of the device (when the output voltage Vout is too low). More specifically, the soft-start circuit described above generates a soft-start voltage Vss (a comparison voltage for soft starting) that starts increasing gently in response to an enabling of a power-on signal EN (an operation permission signal), and a PWM comparator generates a PWM signal having a duty corresponding to the result of comparison of the lower of the error voltage Verr and the soft-start voltage Vss with the slope voltage Vslp (see FIG. 7).
There are other conventional technologies related to the present invention. Patent Publication 2 listed below discloses and proposes a switching power supply control circuit wherein: one of the terminals of a switching power supply control IC is used as a CL terminal for overload protection; an overload protection circuit, which is conventionally connected to a CS terminal for soft starting, is here connected instead to the CL terminal, so as to achieve latch-mode overload protection realized by the turning off of an internal bias source (i.e., the stopping of the output via an OUT terminal); in addition, a comparator or the like for realizing an intermittent oscillation mode is added to the CL terminal. Here, when the pin of the CS terminal for soft starting, which terminal is connected to a PWM comparator, is short-circuited to the pin of the CL terminal for overload protection, the just mentioned circuit added to the CL terminal makes the potential at the CL and CS terminals rise and fall periodically, and thereby makes a main switching device repeatedly perform switching and then stop, thereby achieving intermittent oscillation.
Patent Publication 3 listed below discloses and proposes a DC/DC converter with a protection function including: an input power source; a main switch connected between the input power source and an output terminal and switchable between on and off states; a resistive divider circuit; a difference amplifier-comparator receiving at one input terminal thereof a reference voltage and receiving at the other input terminal thereof a voltage resulting from voltage division by the resistive divider circuit; a selection circuit that applies, to the resistive divider circuit, either an output voltage from the output terminal or an input voltage from the input power source; a PWM comparator connected at one input terminal thereof to the output of the difference amplifier-comparator and connected at the other input terminal thereof the output of a triangular wave generation circuit; a control circuit connected to the output of the PWM comparator and feeding the main switch with a signal for turning it on and off; and means for making the selection circuit feed the input voltage to the resistive divider circuit when the voltage from the input power source is equal to or lower than a predetermined voltage, and for making the selection circuit feed the output voltage from the output terminal to the resistive divider circuit when the voltage from the input power source is higher than the predetermined voltage.
Other conventional technologies directed to power supply devices utilizing soft starting different from that described above are found, for example, in Patent Publication 4 listed below.
Conventionally, in switching regulators that are required to provide high conversion efficiency, to minimize the on-state resistance of a rectifier device, synchronous rectification is adopted in which a synchronous-rectification transistor used as a rectifier device is turned on and off complementarily to an output transistor. Conventional technologies directed to such synchronous-rectification switching regulators are found, for example, in Patent Publications 5 and 6 listed below.
Patent Publication 1JP-A-H07-336999Patent Publication 2JP-A-H09-9616Patent Publication 3JP-A-2004-15881Patent Publication 4JP-A-2003-324941Patent Publication 5JP-A-2003-299348Patent Publication 6JP-A-2003-70238